Melt the butter in a hot skillet over medium heat. Add the onions, peppers and mushrooms. Sauté 3-4 minutes, stirring occasionally, until tender.

Push the vegetables to the side (or remove from the skillet and set aside if your skillet is not big enough) and add the shaved beef to the hot, greased skillet. Using tongs, with the steak marinade sauté the beef and vegetables another 4-5 minutes until the beef is cooked through. Add salt and pepper to taste.

Add the cheese slices to the skillet and remove from heat. Allow the dish to sit a few minutes as the cheese melts.